Signs of a healthy stream
It is important to understand that different streams have different signs of health. We would not expect a steep stream in a mountain canyon to look like one in a broad flat valley. Nor would we expect a stream in a small watershed that dries out to look like a spring-fed stream or a large river. Climate certainly affects vegetation as well as shallow ground water hydrology. However, these characteristics suggest the stream is benefiting from proper functions.
- Vegetation and roots present to protect and stabilize banks
- High water table with lots of water storage
- Good water quality
- More consistent water temperature with shade
- Longer or more consistent flows
- In balance with the water and sediment flowing through the system
- Better fish and wildlife habitat
The stream to the left depends on a broad floodplain to act as a reservoir and a sponge. The stored water and nutrients support plants that are critical for bank stabilization. The stable banks allow the stream to gradually form tight meanders (bends). Meanders allow a stream to decrease water velocities, allowing erosion rates to become very slow. At the same time, the pipe-like active stream channel transports sand and gravel well. It is functioning properly.
Plants that Knit the Soil Together
The deeply and densely rooted water-loving sedge has a tremendous capacity to hold itself in place during a flood event. Its roots knit streambank soils together. These plants grow in places where the abundance of soil water allows these wetland-adapted plants to thrive.
Plants such as many grasses and broad-leaved forbs with weaker root systems grow on higher banks that dry out in summer. These banks may erode more, especially during floods.
How Streams Degrade
Streams are like three-legged stools. They are generally stable if they have a combination of:
- Water at the right times and in the right amounts;
- Landform and soil to spread out floodwaters, dissipate energy, and store water; and
- Vegetation to add roughness and to hold things together.
Take any one of these legs away and the stool tips over - and the stream degrades. How do streams degrade?
- If streams don't flood onto their floodplains, they store little water and erode a lot
- If streams don't retain enough water, they don't grow the right kinds of plants
- If streams don't sustain healthy riparian vegetation, they erode
- If streams erode too much, they lose access to their floodplain.
Once a stream loses one or more of the essential attributes (landform, hydrology or vegetation), it changes. If the changes are limited, natural processes often lead to recovery. If the changes are more severe or coincide with a big event like a flood, a downward spiral leads to channel degradation or channel incision.
After crossing a threshold, streams continue to erode, setting in motion a chain reaction of increased stream velocity, decreased water availability for vegetation at critical times, and considerable loss of valley bottom soil. This can be a pretty ugly process that degrades water quality and most everything else of value in the riparian area.